Mass spectrometric analysis of gas chromatograph fractions is known. It has been recognized that certain classes of organic substances while amenable to mass spectrometric analysis cannot be separated by passing through a gas chromatograph. Therefore, some other means of separating such materials is required as preparation for mass spectrum studies and identification.
A common method for separating aforementioned materials is through liquid chromatography. Liquid chromatography is typically used in analyzing substances comprising large or polar molecules that are unsuitable for gas chromatography.
Liquid chromatography provides a means for separating complex mixtures of either organic or inorganic mixtures into their various components, for example, compounds that are thermally unstable or nonvolatile under normal gas chromatographic conditions.
Another widely used technique for determining structures of chemical species is mass spectrometry. Mass spectrometry identifies an unknown species by comparing its mass spectrum with a reference mass spectrum obtained from a species of known composition. Mass spectrometers generally employ electron impact ionization source for generating ions from the sample material supplied to it.
In liquid chromatography, a chromatographic solvent containing a mixture of components in solution, is passed through a chromatographic column. The chromatographic column separates the mixture, by differential retention in a stationary phase of the column, into its various components. The components emerge from the column as distinct bands in a solvent stream separated in time and therefore distinguishable by the relative retention times. Thus, a liquid chromatograph provides means for sequentially separating individual components from an initially complex mixture which then may be introduced into a detection device, such as a mass spectrometer.
Even though, liquid chromatography provides means for separating a complex mixture into its components, some interfacing means must be provided to remove the liquid chromatograph eluent from these components before their introduction into a detection device, such as a magnetic sector mass spectrometer. Without the removal of the eluent from the component of the mixture before its entry into the ionization chamber of the magnetic sector mass spectrometer, the mass spectra obtained therefrom cannot be used for precise identification of the compounds present in the component.
The organic liquids used as eluents in liquid chromatograph, if present even in minute amounts, constitute a major source of error in any subsequent mass spectrometric analysis. Such an error occurs because the eluate exiting from the liquid chromatographic column generally contains the component in the range of about 10-100 parts per million (ppm). If one were to directly introduce the eluate containing the component into the ionization chamber of the magnetic sector mass spectrometer, the detection system of the magnetic sector mass spectrometer will be overwhelmed by the eluent and detection of the component may not occur. Additionally, the vacuum system of the magnetic sector mass spectrometer will be inundated by the eluent. As a result, an interfacing means and method which removes the eluent while efficiently transferring the material of interest to the magnetic sector mass spectrometer is needed and is provided by this invention.
However, conventional mass spectrometers having the electron impact mode of ionization have limited applications. One of the shortcomings of conventional electron impact mass spectrometry is that many types of compounds give a very weak signal for the molecular ion being analyzed, even when the molecular ion has as high as 1 or 2% relative abundance. This often means that a significantly higher quantity of the sample is required for determining its molecular weight. As a result, chemical ionization mass spectrometry has recently emerged as an important new technique to obtain additional information not provided by electron impact methods.
The extensive molecular fragmentation observed in the electron impact spectra of many compounds results from the fact that during the initial electron/molecule interaction, many molecules receive considerable energy above the ionization voltage. Typically, the molecule ion undergoes one or more bond breaks thereby reducing the intensity of the parent ion. As a result it is difficult to determine with certainty the molecular weight of the parent ion on the basis of the electron impact spectra.
When compared, a chemical ionization mass spectrum obtained from an unfragmented parent molecule provides fairly precise information about the molecular weight of the parent molecule being analyzed. In addition, the chemical ionization fragmentation patterns may differ sufficiently from the electron impact patterns to reveal other structural features not indicated by the conventional mass spectrum.
One of the major problems encountered in connecting liquid chromatograph/mass spectrometer (LC/MS) interfaces to magnetic sector mass spectrometers having chemical ionization sources is the presence of a very high voltage associated with the ion acceleration process. The high voltage between the chemical ionization source and a conventional LC/MS interface can result in an electrical glow discharge. This glow discharge produces a conductive path, which is very damaging to the highly sensitive equipments used in mass spectrometry.
Dorn et al. in U.S. Pat. No. 4,980,057 disclose the use of a nebulizer having a combination ultrasonic/pneumatic nebulizing means, the use of a heater directly in the gas stream of the evaporation chamber and the control of this heater using a thermocouple located near the inlet of the nozzle, the use of a momentum separator in which the skimmers are symmetrically pumped from two directions in order to minimize turbulence and the use of a three-stage momentum separator which produces significantly low pressures at the magnetic sector mass spectrometer while maintaining a high yield of sample particles. The use of an ultrasonic nebulizer gives much greater flexibility compared to other designs because there is no need to readjust the nebulizer temperature when solvents change (gradient elution techniques) as with prior art thermospray nebulizers. Further, using an ultrasonic nebulizer, the inert gas flow may be adjusted at will to accommodate changing liquid chromatograph flow rates and solvent volatility. However, no apparatus or method for connecting a liquid chromatograph to a magnetic sector mass spectrometer having a chemical ionization source therein is disclosed.
Another interface device utilizing particle beam technology is currently marketed by Hewlett-Packard Company. This device, disclosed in U.S. Pat. No. 4,863,491 to Brandt et al., uses a pneumatic nebulizer and a two stage momentum separator, The stated sensitivity specification for the Hewlett-Packard device is a signal/noise ratio of 50:1 on the molecular ion of caffeine using a sample size of 20.times.10.sup.-9 g and an LC flow of 0.5 ml/min. methanol.